Looping during a commercial flight

"LordAvalon" wrote in message
om...
"Rob Stokes" wrote in message
...
I have no proof, but looping an airliner without the passengers being
aware
is not possible. Perhaps visually the passengers wouldn't notice, but
balance wise / spatially they certainly would. I have heard from
many
sources that the last airliner to be taken for a loop was a B-707
after
which many of the A/C components shifted up to an inch, the pilot was
immediately disciplined! other similar instances have occurred on
Russian
airliners by Ex-military pilots shortly after the war. At the
Goodwood
Festival Of Speed (England) this year, there was a 'semi' aerobatic
display
by a south African jumbo (consisting of very low steep turns)

Thanks for your input.
I am not an expert in flying terms so maybe it is not exactly
"looping" i heard of. The plane would drop to the left for exemple
with his longitudinal axis straight and acquire vertical speed. Then
the pilot would shift the stick right very slowly to convert this
vertical speed in a centrifugal vector. This is supposed to recreate
the same phenomemom as a pail full of water tied to a rope you turn
fast. So, as this theory goes, after the initial banking the figure
would make the passengers feel only a moderate or no increase in their
weight, the force being directed towards the floor of the plane. Since
the long axis of the plane is not disturbed no other noticeable
effects should appear...

But if it is unheard of, it may well be purely theorical or simply an
urban legend!

No. It's possible, but I haven't heard of it being done successfully in
the pure vertical plane as in a straight positive g loop. More likely in
a three dimensional maneuver like a barrel roll. The energy or Ps bleed
for a big jet is tremendous in the vertical plane, and an entry airspeed
required to get everything up and around the topside apex would be
considerable to say the least. Considering the huge drag index against
what would have to be at least a 4 positive g pull to even have a chance
at an inverted apex, the entry speed would be prohibitive for safety I
think; considering the aerodynamic factors and the requirement for
positive g at the top for the scavenger pumps. In other words, floating
it through the top unloaded is eliminated from the equation because of
the pumps if nothing else. The engines oil system requires positive g to
operate. Unloading would deny the engines oil and the result at a high
percent RPM or EPR could lose you the engines. The result of that would
be a negative nose rate and developing vertical descent while inverted
that could really spoil your afternoon :-)
In a normal loop, you ease off the positive g at the top to round it off
and keep the AOA in limits. In a big jet, this could really be a
problem.
A three dimensional roll (barrel roll) is possible for a big jet, and
has been done, even in the prototype 707. You are right about keeping
the g positive all the way around. Both myself and several other
demonstration pilots at one time or another have barreled airplanes with
a glass of water on the instrument glare shield. I have done it in a D18
Twin Beech several times and Bob Hoover has done it in a Shrike
Commander......many times. :-)
In barrel rolling a large airplane, you can keep the stress at a minimum
and perform the maneuver in a wide low positive g profile that is kept
positive all the way around. Because it's FLOWN around the circle rather
than a pure roll on the longitudinal axis, the deep dishout on the back
side that would be the result of an aileron roll can be eliminated by
the trade off in altitude on the upside against the backside recovery.
Basically, even though I'm sure a pure loop has been done by heavy jets
at one time or another, I would classify the maneuver as extremely risky
at best, and totally unwise to attempt. But as I said....I'm sure
somebody managed to get the entry speed necessary to do it regardless of
the risk, and somehow managed to pull it around without shedding all the
critical parts in the process. But I wouldn't want to be the next pilot
to fly that airplane :-)
Dudley Henriques
International Fighter Pilots Fellowship
Flight Instructor/Aerobatics/Retired